Chapter 6: Fog Seals

2.0 Fog Seal Project Selection

Fog seals are used as a method of enrichment of a pavement surface and as a method of holding stone in place.  Thus, they are suitable to treat raveled and aged pavements.  There is, at the present time, no simple way of quantifying the degree of aging in a pavement other than by visual inspection. Different asphalts will age at different rates and experience of individual districts is key to determining treatment timing.  Some modified asphalts such as asphalt rubber and polymer modified asphalts will age at a slower rate than conventional binders. Fog seals will not correct distresses such as cracking, base failures, excessive stone already lost, or any other severe pavement structural defects.

On the traveled way, fog seals should only be used where surface penetration of the emulsion can be expected; that is, aged and raveled hot mix surfaces, chip-sealed surfaces, and open graded asphalt surfaces.  On shoulders, gores, or dikes, penetration is desirable, but it is not essential.  Fog seals darken the pavement surface and create distinct demarcation in these regions.

In general, traffic level is not a determining factor except in job set up.  For situations requiring that the sealed pavement be opened to traffic shortly after the application of the seal, a blotter coat of sand may be used to prevent pick-up.

2.1 Old or Damaged Surfaces

All asphalts harden as they age, primarily due to oxidation, volatile loss and other aging mechanisms (4).  Hardening of an asphalt film takes place at different rates according to the access of air and temperature conditions in the pavement.  Permeable pavements or pavements with high void contents can therefore age faster.  Water ingress can also carry dissolved oxygen and trace elements that may promote aging.  This means that pavements with open surfaces tend to age faster than those with closed surfaces.  However, if modified binders are used (e.g., asphalt rubber, polymer modified asphalt), the thicker films created by the higher binder content reduce the rate of aging.

Aging results in a binder that is more brittle.  These binders eventually experience cohesive binder failures under traffic loads and stone loss or raveling.  In some cases, the asphalt produces oxidized compounds that are acidic and bond well to the aggregate; however, these compounds may also react with water causing adhesive failure or stripping.

Fog seal use on the traveled way should generally be limited to only those locations having an open surface texture.  This includes chip seals, heavily aged dense graded and open graded.  However, the seal may fill voids and reduce or eliminate the drainage function of Open-Graded Friction Coarse (OGFC).  Figure 2 shows a typical fog seal application, while Figures 3 through 5 show a range of suitable and unsuitable surfaces for fog seal project selection.  The results of good fog seal applications are shown in Figures 6 and 7.  It is important to always check the application rate and ensure that the emulsion has been diluted correctly.

Figure 2: Fog Seal Application	Figure 3: Suitable Surface, Heavily Aged Dense Graded HMA
Figure 4: Unsuitable Surface, Dense Graded HMA With Closed Surface	Figure 5: Suitable Surface, Open Graded HMA
Figure 6: Chip Seal Before and After Fog Seal

Fog seals (with sand blotter coats) may be used as a pavement maintenance treatment on lower speed roads or low traffic volume roads and shoulders. This protects the hot mix asphalt or chip seal surface.  In some instances (where traffic is straight), a fog seal with a blotter coat may also be acceptable. The sand will generally be removed by the traffic leaving a good surface texture.

2.2 New Surfaces

Flush coats (fog seals with light sanding) are used as a construction seal for new chip seals to lock the chips in place.  This reduces vehicle/windshield damage due to flying chips when traffic is allowed on the new seal.  These fog seals with sand blotter coats may also be used as a pavement maintenance treatment on lower speed roads or low traffic volume roads.  This protects the hot mix asphalt or chip seal surface.

Fog seals are also suitable for sealing new shoulders, gores, or dikes.  During construction on milled or ground HMA surfaces, fog seals may be used to keep dust down and prevent rock loss before the next surface is placed.

2.3 Surface Protection

Fog seals may be used to protect a hot-mix asphalt (HMA) surface that is not aged significantly (i.e., within 1-2 years of placement after a major rehabilitation or maintenance treatment).  This creates a layer of asphalt that seals surface voids and prevents air and water ingress.  Do not seal any pavement less than one year old unless the pavement is showing severe raveling resulting from an oil shortage in the mix.

2.4 Fog Seal Performance – Benefits and Limitations

Fog seals are an inexpensive way of arresting raveling and adding binder back into aged surfaces.  They can also hold chips in place in fresh chip seals, (or older chip seals beginning to lose aggregate) reducing the potential for vehicle damage. 

Fog seals are not useful as seal coats on tight surfaces without the addition of aggregates as they will reduce surface texture and may create a slippery surface.  Fog seals should not be used on Chemically Modified Crumb Rubber Asphalt (CMCRA) Rubberized Asphalt Concrete (RAC) or polymer modified mixes unless the pavements are over five years old as these binders age at a slower rate. 

The application of fog seals is also limited by weather.  A cut off date in the fall (e.g. September 1st) will ensure that rain will not be a factor and that the emulsion will fully cure before freezing conditions are encountered.  In addition, seal coats applied in the winter have less time to penetrate the pavement and are more prone to cause slick surface conditions.